Methods and apparatus for multi-carrier communication systems with adaptive transmission and feedback

ABSTRACT

An arrangement is disclosed where in a multi-carrier communication system, the modulation scheme, coding attributes, training pilots, and signal power may be adjusted to adapt to channel conditions in order to maximize the overall system capacity and spectral efficiency without wasting radio resources or compromising error probability performance.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional PatentApplication No. 60/544,521, filed on Feb. 13, 2004. This applicationalso relates to PCT Application No. PCT/US05/03518 titled “Methods andApparatus for Overlaying Multi-Carrier and Direct Sequence SpreadSpectrum Signals in a Broadband Wireless Communication System,” filedJan. 27, 2005, which claims the benefit of U.S. Provisional ApplicationNo. 60/540,032 filed Jan. 29, 2004 and U.S. Provisional Application No.60/540,586 filed Jan. 30, 2004.

BACKGROUND

Adaptive modulation and coding (AMC) has been used in wireless systemsto improve spectral efficiency in a fading environment where signalquality varies significantly. By adjusting the modulation and codingscheme (MCS) in accordance with the varyingsignal-to-interference-plus-noise ratio (SINR), reliable communicationlink can be maintained between communicating devices. For example, inCDMA2000 1xEV-DO system, twelve different modulation/coding schemes areprovided. AMC is also used in CDMA2000 1xEV-DV and 3GPP HSDPA systems.

To improve performance, in addition to the MCS, other system functionssuch as channel estimation, transmission power control (TPC), andsubchannel configuration can be adjusted in accordance with the state ofthe communication channel. For example, channel estimation typicallyutilizes training symbols or pilot data, which are known to both thetransmitter and the receiver. For coherent modulation, the channelinformation can be extracted at the receiver by comparing the pilots andtheir corresponding received versions. For non-coherent modulation, thereceived samples of the pilots are used as reference for the detectionof the transmitted data.

Channel estimation is an important part of multi-carrier (MC)communication systems such as Orthogonal Frequency Division Multiplexing(OFDM) systems. In conventional OFDM systems, such as IEEE802.11a,802.11g, 802.16, or DVB-T system, pilots are transmitted for channelestimation. The pilots are fixed and form part of other functions suchas MCS, TPC, and subchannel configuration in some wireless systems.

Fast TPC can compensate for fast fading. In a multi-cell multiple-accesssystem, TPC is also used to reduce intra-cell and inter-cellinterference and to conserve battery life for the mobile station bytransmitting with only necessary power. TPC is one of many functions insome wireless systems, along with MCS, pilot attributes, subchannelconfiguration, etc.

The subchannel configuration is normally defined and fixed in anoperation, and it is usually not considered an adjustable function ofthe system to be adapted to the user profile and/or operationalenvironment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representative cellular communication system.

FIG. 2 is a basic structure of a multi-carrier signal in the frequencydomain, made up of subcarriers.

FIG. 3 depicts a radio resource divided into small units in bothfrequency and time domains: subchannels and time slots.

FIG. 4 is an illustration of a control process between Device A andDevice B, each of which can be a part of a base station and a mobilestation depicted in FIG. 1.

FIG. 5 illustrates a joint adaptation process at a transmitter of anOFDM system which controls coding, modulation, training pilot pattern,and transmission power for a subchannel.

FIG. 6 is an illustration of a control messaging associated with datatransmission between communication devices.

FIG. 7 illustrates two different training pilot patterns plotted for amulti-carrier system.

FIG. 8 illustrates a power control in AMCTP scheme for an OFDM system.

DETAILED DESCRIPTION

Methods and apparatus for adaptive transmission of wirelesscommunication signals are described, where MCS (modulation and codingscheme), coding rates, training pilot patterns, TPC (transmission powercontrol) levels, and subchannel configurations are jointly adjusted toadapt to the channel conditions. This adaptation maximizes the overallsystem capacity and spectral efficiency without wasting radio resourcesor compromising error probability performance.

Furthermore, the subchannel composition is designed to be configurableso that it can be adjusted statically or dynamically according to theuser profiles or environmental conditions. The methods for obtaining thechannel information and for transmitting the control information in thejoint adaptation scheme are also described below, such as feedback ofchannel condition and indexing of the joint scheme, along with methodsfor reducing the overhead of messaging.

The mentioned multi-carrier system can be of any special format such asOFDM, or Multi-Carrier Code Division Multiple Access (MC-CDMA) and canbe applied to downlink, uplink, or both, where the duplexing techniqueis either Time Division Duplexing (TDD) or Frequency Division Duplexing(FDD).

The apparatus and methods are described with respect to variousembodiments and provide specific details for a thorough understandingand enablement. One skilled in the art will understand that theinvention may be practiced without such details. In some instanceswell-known structures and functions are not shown or described in detailto avoid unnecessarily obscuring the description of the embodiments.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” Words using the singular or pluralnumber also include the plural or singular number respectively.Additionally, the words “herein,” “above,” “below” and words of similarimport, when used in this application, shall refer to this applicationas a whole and not to any particular portions of this application. Whenthe claims use the word “or” in reference to a list of two or moreitems, that word covers all of the following interpretations of theword: any of the items in the list, all of the items in the list and anycombination of the items in the list.

The content of this description is applicable to a communication systemwith multiple transmitters and multiple receivers. For example, in awireless network, there are a number of base stations, each of whichprovides coverage to its designated area, typically called a cell.Within each cell, there are mobile stations. FIG. 1 illustrates acommunication system that is representative of such a system, where BaseStation 110 is communicating with Mobile Stations 101 and 102 in SectorA of its cell site while Base Station 120 is communicating with MobileStations 103, 104, and 105 in Sector B of its cell site.

A multi-carrier multiple-access system is a special case of generalcommunication systems and hereinafter is employed as a representativecommunication system to describe the embodiments of the invention.

Multi-Carrier Communication System

The physical media resource (e.g., radio or cable) in a multi-carriercommunication system can be divided in both the frequency and the timedomains. This canonical division provides a high flexibility and finegranularity for resource sharing.

The basic structure of a multi-carrier signal in the frequency domain ismade up of subcarriers. Within a particular spectral band or channel,there are a fixed number of subcarriers, and there are three types ofsubcarriers:

-   -   1. Data subcarriers, which carry information data;    -   2. Pilot subcarriers, whose phases and amplitudes are        predetermined and made known to all receivers and which are used        for assisting system functions such as estimation of system        parameters; and    -   3. Silent subcarriers, which have no energy and are used for        guard bands and DC carrier.

The data subcarriers can be arranged into groups called subchannels tosupport scalability and multiple-access. The carriers forming onesubchannel are not necessarily adjacent to each other. Each user may usepart or all of the subchannels. The concept is illustrated in FIG. 2,which is the basic structure of a multi-carrier signal in the frequencydomain, made up of subcarriers. Data subcarriers can be grouped intosubchannels in a specified manner. The pilot subcarriers are alsodistributed over the entire channel in a specified manner.

The basic structure of a multi-carrier signal in the time domain is madeup of time slots to support multiple-access. The resource division inboth the frequency and time domains is depicted in FIG. 3, which is theradio resource divided into small units in both the frequency and timedomains (subchannels and time slots). The basic structure of amulti-carrier signal in the time domain is made up of time slots.

Adaptive Transmission and Feedback

The underlying principles of adaptive transmission and feedback are bothto increase the degree of freedom of a transmission process and tosupply information for the adaptation process of a communication system.The adaptation process adjusts the allocated modulation schemes, codingrates, pilot patterns, power levels, spatial processing schemes,subchannel configurations, etc. in accordance with the transmissionchannel state and condition, for improving system performance and/orcapacity.

Below, AMCTP (adaptive modulation, coding, training and power control)is used as a general term, where its variations can be applied toappropriate applications. There are different adaptive transmissionschemes that are subsets of the AMCTP scheme, such as AMCT (adaptivemodulation, coding and training), AMTP (adaptive modulation, training,and power control), AMT (adaptive modulation and training), and soforth.

FIG. 4 is an illustration of the control process between Device A andDevice B, each of which can be a part of a base station and a mobilestation depicted in FIG. 1, during adaptive transmission. Thetransmitter 401 of Device A transmits data 402 and associated controlinformation 404 to Device B, based on an output of the adaptationprocess 406. After a receiver 408 of Device B receives the transmitteddata 402 and control information 404, a measurement process 410 ofDevice B measures a channel conditions and feeds a channel qualityinformation (CQI) 412 back to Device A.

The granularity of AMCTP schemes in a multi-carrier system can beuser-based, where one or multiple subchannels may be used, or thegranularity can be subchannel-based, where a subchannel may contain oneor more subcarriers. Likewise, the granularity of CQI can be user- orsubchannel-based. Both AMCTP and CQI may change over time and may differfrom one time slot to another.

FIG. 5 illustrates a joint adaptation process at a transmitter of anOFDM system which employs separate processing block to control thecoding 502, modulation 504, training pilot pattern 506, and transmissionpower for a subchannel 508. Each block may be implemented combined orseparately in circuitry, in dedicated processors, in a digital signalprocessor, as a microprocessor implemented subroutine, etc.

FIG. 6 is an illustration of control messaging associated with the datatransmission between communication devices, such as Device A and B inFIG. 4. In FIG. 6 the AMCTP indicator 602 is associated with datatransmission 604 on a forward link from the transmitter to the receiver,and CQI 606 is associated with the information feedback from thereceiver to the transmitter on a return channel.

In a system where AMCTP is used, the transmitter relies on the CQI toselect an appropriate AMCTP scheme for transmitting the next packet, orretransmitting a previously failed packet, required in an automaticrepeat request (ARQ) process. The CQI is a function of one or more ofthe following: received signal strength; average SINR; variance in time;frequency or space; measured bit error rate (BER); frame error rate(FER); or mean square error (MSE). Channel conditions hereinafter arereferred to as one or more of the following, for a user or a subchannel:signal level, noise level, interference level, SINR, fading channelcharacteristics (Doppler frequency, delay spread, etc.), or channelprofile in time or frequency domain. The detection of the channelcondition can be at the transmitter, the receiver, or both.

An MCS in AMCTP is referred to as a modulation and error correctioncoding scheme used in the system. By matching an MCS to a specificchannel condition (e.g., SINR level), a better throughput is achieved.Varying only the MCS is a sub-optimal approach since other factors suchas training pilot patterns or subchannel compositions also impact systemperformance.

A pilot pattern includes the number of (training) pilot symbols, thelocation of the symbols in time/frequency/space, the amplitude andphase, and other attributes of these symbols. The system may usedistinctive pilot patterns to suit different MCS and channel conditions.The pilot pattern requirements for a robust channel estimation vary withthe SINR of the channel and the channel profile.

In a multi-carrier system, pilots are transmitted on certain positionsin the time-frequency grid. FIG. 7 illustrates two of many differenttraining pilot patterns that may be used, each plotted for amulti-carrier system, where the dark shaded time-frequency grids 702 areallocated as training pilot symbols. One criterion for choosing a pilotpattern is that the pilot assisted channel estimation should not be abottleneck for the link performance, and that the pilot overhead shouldbe kept to a minimum. The joined adaptation of training pilot patterntogether with MCS is a more effective way of matching the channelconditions, and results in a better performance compared with a mereadaptation of MCS.

The power control information may include an absolute power level and/ora relative amount to increase or decrease the current power setting. Ina multi-carrier system, the power levels of different subchannels areset differently such that minimum power is allocated to a subchannel tosatisfy its performance requirements while minimizing interference toother users.

The power control can be user- or subchannel-based. FIG. 8 is anillustration of a power control in an OFDM system where digital variablegains 802 G1, G2 . . . GN are applied to subchannels 804 that may havedifferent MCSs with different transmission power levels. Analog domaingain 806 Ga is used to control the total transmission power signalprocesses to meet the requirements of the transmission power of thedevice. In FIG. 8, after variable gains are applied to subchannels 804,they are inputted to the inverse discrete Fourier transform (IDFT)module. The outputs from the IDFT are the time domain signals, which areconverted from parallel to sequential signals after a cyclic prefix isadded to them.

Table 1 is an example of a general AMCTP table (or CQI table). It shouldbe noted that some pilot patterns in the table can be the same. Thetotal number of indexes used to represent different combinations of thejoint adaptation process can be different for AMCTP index and CQI index.For instance, it is not necessary to send absolute transmission powerinformation to the receiver(s). Some AMCTP information, such as relativepower control or code rate, can be embedded in the data transmissioninstead of being conveyed in the AMCTP index.

TABLE 1 An example of general AMCTP. Code Training Transmit IndexModulation Rate Pilot Power 1 QPSK 1/16 Pattern 1 + 2 QPSK ⅛ Pattern 2 +3 QPSK ¼ Pattern 3 + 4 QPSK ½ Pattern 4 + 5 QPSK ½ Pattern 5 + 6 16QAM ½Pattern 6 + 7 16QAM ½ Pattern 7 + 8 16QAM ¾ Pattern 8 + 9 16QAM ¾Pattern 9 + 10 64QAM ⅔ Pattern 10 + 11 64QAM ⅚ Pattern 11 + 12 64QAM ⅚Pattern 12 Max-1x 13 64QAM ⅚ Pattern 13 Max-2x 14 64QAM ⅚ Pattern 14Max-3x

In a general AMCTP or CQI table, different training pilot patterns maybe used for different modulations and code rates. However, even for thesame modulation and coding, different patterns can be used to matchdifferent channel conditions. In order to make the table more efficient,more indexes can be allocated to the more frequently used scenarios. Forexample, several training pilot patterns can be allocated to the sameMCS that is used more frequently, to achieve finer granularity and thushave a better match with different channel conditions.

Table 2 is a simple realization of the AMCTP index or the CQI index. Inone embodiment, as shown in Table 2, the AMCTP and CQI index is Graycoded so that one bit error in the index makes the index shift to theadjacent index.

In some cases, a different number of pilot symbols is used for the sameMCS. In one embodiment, to keep the packet size the same when the sameMCS is used with a different number of pilot symbols, rate matchingschemes such as repetition or puncturing is employed. For instance inTable 2, for Index 010 and Index 011, Pattern 3 has more pilot symbolscompared to Pattern 2. The code rate of Index 010 is ½, which ispunctured to 7/16 for Index 011 to accommodate the extra pilot symbols.In one embodiment, more significant bits in the CQI index are protectedwith stronger error protection code on the return channel.

TABLE 2 Another example of AMCTP or CQI table. Index (Gray Code TrainingTransmit coded) Modulation Rate Pilot Power 000 QPSK ¼ Pattern 1 Max 010QPSK ½ Pattern 2 Max 011 QPSK 7/16 Pattern 3 Max 001 16QAM ½ Pattern 2Max 101 16QAM 7/16 Pattern 3 Max 111 64QAM ⅔ Pattern 2 Max 110 64QAM ⅚Pattern 3 Max 100 64QAM ⅚ Pattern 3 Max-X

Other factors that can be used in the adaptation process includemodulation constellation arrangements, transmitter antenna techniques,and subchannel configuration in a multi-carrier system.

For some modulation schemes such as 16QAM and 64QAM, how informationbits are mapped to a symbol determines the modulation schemes'reliability. In one embodiment, constellation arrangement is adjusted inthe adaptation process to achieve a better system performance,especially during retransmission in a hybrid ARQ process.

Some multiple antenna techniques, such as transmission diversity, areused to improve the transmission robustness against fading channeleffects, whereas other multiple antenna techniques such asmultiple-input multiple-output (MIMO) schemes are used to improvetransmission throughput in favorable channel conditions. In oneembodiment of the adaptive transmissions the antenna technique used fora transmission is determined by the adaptation process.

In a multi-carrier multi-cell communication system, when all subcarriersin one subchannel are adjacent or close to each other, they are morelikely to fall in the coherent bandwidth of a fading channel; thus theycan be allocated to users that are either fixed in location or are moveslowly. On the other hand, when subcarriers and/or subchannels thatbelong to one user are scattered in the frequency domain, it results inhigher diversity gains for the fast moving users, and a betterinterference averaging effect.

Given the fact that different configurations of subchannel compositionsare suitable for different scenarios or user profiles, subchannelconfiguration is included in the transmission adaptation process. In oneembodiment, the subchannel configuration information is broadcast on thecommon forward control channel to all users such that each user isinformed of its subchannel configuration.

In another embodiment, the subchannel configuration is adjustedaccording to deployment scenarios. For instance, when a base station isnewly deployed with less interference, one form of subchannelconfiguration is used, and when more users join the network or moreadjacent base stations are deployed, which results in strongerinterference to the users in the system, a different subchannelconfiguration with better interference averaging effect is used.

The following paragraphs describe a method of transmitting the controlmessage between the transmitter and receiver, when the AMCTP scheme isimplemented. A forward control link is defined here as the transmissionof the AMCTP indicator from the transmitter to the receiver, and areturn control channel is defined as the transmission of CQI, as thefeedback information, from the receiver to the transmitter, as shown inFIG. 4.

The AMCTP indicator on the forward link can be sent either separately orjointly. For instance, the power control information, training pilotpattern indicator, or antenna diversity scheme can be embedded in thedata transmission. In another embodiment, AMCTP is transmitted on aseparate control channel with stronger error protection.

One way for the transmitter to obtain CQI is to have it explicitly sentfrom the receiver to the transmitter based on channel conditionmeasurements at the receiver during previous transmission(s). The CQI isthen used by the transmitter to determine what AMCTP scheme to use forthe next transmission. In one embodiment, CQI for one user isperiodically updated on the return channel, even when there is noforward transmission targeted for that user. In this case the receivermeasures the channel conditions from the common broadcast transmissionor the data transmission targeted to other users.

In one embodiment, the transmitter or the receiver uses any of severalknown predictive algorithms to predict current or future channelconditions based on previous channel measurements. This is moreeffective for a fast fading environment where the past measurements maynot match the current transmission closely, due to the fast channelvariations. The output of the predictive algorithm is then used by theadaptation process to select the best possible scheme for the currenttransmission.

Another method to obtain CQI is through the transmission of a probingsequence from the receiver to the transmitter on the return channel. Inone embodiment, in a multi-carrier communication system, a probingsequence is transmitted from the receiver to the transmitter using anoverlay scheme where the probing sequence is overlaid to the datatraffic without having negative impact on the data transmissionperformance. In this case the transmitter estimates the channel profilein the time and/or frequency domains based on the received probingsequence. This is especially effective for TDD systems due to thereciprocity of the channel conditions on forward and reverse channels.

The AMCTP indicator or CQI can be sent per user or per subchannel. Inone embodiment if per subchannel feedback is employed, since the AMCTPand CQI information for the same users are highly correlated, first thesource coding is employed to compress the CQI, and then the errorcorrection coding is applied to the compressed CQI to provide sufficienterror protection.

In another embodiment, in hybrid ARQ retransmission, the transmitter maynot use the requested CQI for the retransmission, while it may use therequested CQI for a new packet transmission. Instead, in thisembodiment, it selects an AMCTP scheme that is appropriate for theretransmission at the same power level as in the previoustransmission(s), in order to reduce interference with other users.

It should be pointed out that the AMCTP index used for the transmissionfrom the transmitter to the receiver may be different from the CQI thatthe receiver requested, because the transmitter may have otherconsiderations such as quality of service (QoS) for different users,network traffic load, and power allocation limit.

The above detailed description of the embodiments of the invention isnot intended to be exhaustive or to limit the invention to the preciseform disclosed above or to the particular field of usage mentioned inthis disclosure. While specific embodiments of, and examples for, theinvention are described above for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. Also, the teachingsof the invention provided herein can be applied to other systems, notnecessarily the system described above. The elements and acts of thevarious embodiments described above can be combined to provide furtherembodiments.

All of the above patents and applications and other references,including any that may be listed in accompanying filing papers, areincorporated herein by reference. Aspects of the invention can bemodified, if necessary, to employ the systems, functions, and conceptsof the various references described above to provide yet furtherembodiments of the invention.

Changes can be made to the invention in light of the above “DetailedDescription.” While the above description details certain embodiments ofthe invention and describes the best mode contemplated, no matter howdetailed the above appears in text, the invention can be practiced inmany ways. Therefore, implementation details may vary considerably whilestill being encompassed by the invention disclosed herein. As notedabove, particular terminology used when describing certain features oraspects of the invention should not be taken to imply that theterminology is being redefined herein to be restricted to any specificcharacteristics, features, or aspects of the invention with which thatterminology is associated.

In general, the terms used in the following claims should not beconstrued to limit the invention to the specific embodiments disclosedin the specification, unless the above Detailed Description sectionexplicitly defines such terms. Accordingly, the actual scope of theinvention encompasses not only the disclosed embodiments, but also allequivalent ways of practicing or implementing the invention under theclaims.

While certain aspects of the invention are presented below in certainclaim forms, the inventors contemplate the various aspects of theinvention in any number of claim forms. Accordingly, the inventorsreserve the right to add additional claims after filing the applicationto pursue such additional claim forms for other aspects of theinvention.

1. A communication method in an OFDM (Orthogonal Frequency DivisionMultiplexing) system, the method comprising: adapting modulationschemes, coding rates, pilot patterns, and power levels of a signal toproduce an adapted signal, wherein: the adaptation process is performedin a transmitter; and the adaptation process is based on channel stateinformation; generating, by the transmitter, a control informationsignal representing adaptation information, wherein the controlinformation signal includes a single index value that refers toadaptation details of a modulation scheme, coding rate, pilot pattern,and power level; transmitting the adapted signal and the controlinformation signal by the transmitter, separately or jointly; receivingthe adapted signal and the control information signal by a receiver;looking-up the adaptation details from a look-up table at the receiverusing the index value, wherein: the table includes multiple sets ofadaptation schemes employed by the transmitter; and the number of pilotpatterns associated with a particular combination of a modulation schemeand a coding rate is a function of how often the particular combinationis used for transmission; measuring the channel state by the receiver,based on the received signals, wherein the measurements include: signalstrength, average SINR (signal to interference plus noise ratio),variance in time, variance in frequency, variance in space, BER (biterror rate), FER (frame error rate), MSE (mean square error), or anycombination thereof; and forming a probing sequence using the channelstate measurements, computed channel quality information, or channelstate measurements and computed channel quality information.
 2. Themethod of claim 1, wherein the channel quality information based onchannel state measurements during a previous transmission is used by atransmitter to determine an adaptation scheme for the next transmission,or wherein for a retransmission the transmitter selects an adaptationscheme from one of the previous transmissions that is appropriate forthe retransmission.
 3. The method of claim 1, wherein the channel statemeasurements for a receiver is periodically updated on a feedbackchannel even when there is no forward transmission targeted to thatreceiver and wherein a receiver measures channel state from a commonbroadcast transmission or data transmission targeted to other receivers.4. The method of claim 1, wherein the probing sequence is transmittedfrom the receiver to the transmitter using an overlay scheme in whichthe probing sequence is overlaid on a data transmission and thetransmitter estimates a channel profile in the time domain, thefrequency domain, or the time and frequency domains based on thereceived probing sequence.
 5. A communication method in an OFDM(Orthogonal Frequency Division Multiplexing) receiver, the methodcomprising: receiving an adapted signal from a transmitter, thetransmitter adapting modulation schemes, coding rates, pilot patterns,and power levels of the signal based on channel state information;receiving a control information signal from the transmitter, the controlinformation signal including an index value that refers to adaptationdetails of a modulation scheme, coding rate, pilot pattern, and powerlevel used by the transmitter; looking-up the adaptation details from alook-up table at the receiver using the index value, wherein: the tableincludes multiple sets of adaptation schemes employed by thetransmitter; and the number of pilot patterns associated with aparticular combination of a modulation scheme and a coding rate is afunction of how often the particular combination is used fortransmission; measuring the channel state based on the received signals,wherein the measurements include: signal strength, average SINR (signalto interference plus noise ratio), variance in time, variance infrequency, variance in space, BER (bit error rate), FER (frame errorrate), MSE (mean square error), or any combination thereof; and forminga probing sequence using the channel state measurements, computedchannel quality information, or channel state measurements and computedchannel quality information.
 6. The method of claim 5, wherein thechannel state measurements for a receiver is periodically updated on afeedback channel even when there is no forward transmission targeted tothat receiver and wherein a receiver measures channel state from acommon broadcast transmission or data transmission targeted to otherreceivers.
 7. The method of claim 5, wherein the probing sequence istransmitted from the receiver to the transmitter using an overlay schemein which the probing sequence is overlaid on a data transmission.
 8. Themethod of claim 7, wherein the transmitter estimates a channel profilein the time domain, the frequency domain, or the time and frequencydomains based on the received probing sequence.